The present invention relates generally to an apparatus and method for measuring nuclear magnetic resonance properties of an earth formation traversed by a borehole, and more particularly, to an apparatus and method for generating a pulse sequence.
It is well recognized that atomic particles of an earth formation having non-zero nuclear spin magnetic moment, for example protons, have a tendency to align with a static magnetic field imposed on the formation. Such a magnetic field may be naturally generated, as is the case for the earth's magnetic field, B.sub.E. An RF pulse applying a second magnetic field transverse to B.sub.E creates a magnetization component in the transverse plane as (perpendicular to B.sub.E) which precesses about the B.sub.E vector with a characteristic resonance known as the Larmor frequency, .omega..sub.L, which depends on the strength of the static magnetic field and the gyromagnetic ratio of the particle. Hydrogen nuclei (protons) precessing about a magnetic field B.sub.E of 0.5 gauss, for example, have a characteristic frequency of approximately 2 kHz. If a population of hydrogen nuclei were made to precess in phase, the combined magnetic fields of the protons can generate a detectable oscillating voltage in a receiver coil, conditions known to those skilled in the art as free induction decay or a spin echo. Hydrogen nuclei of water and hydrocarbons occurring in rock pores produce nuclear magnetic resonance (NMR) signals distinct from signals arising from other solids.
U.S. Pat. No. 4,717,878 issued to Taicher et al. and U.S. Pat. No. 5,055,787 issued to Kleinberg et al., describe NMR tools which employ permanent magnets to polarize hydrogen nuclei and generate a static magnetic field, B.sub.0, and RF antennas to excite and detect nuclear magnetic resonance to determine porosity, free fluid ratio, and permeability of a formation. The atomic nuclei align with the applied field, B.sub.0, with a time constant of T.sub.1. After a period of polarization, the angle between the nuclear magnetization and the applied field can be changed by applying an RF field, B.sub.1, perpendicular to the static field B.sub.0, at the Larmor frequency f.sub.L =.gamma.B.sub.0 /2.pi., where .gamma. is the gyromagnetic ratio of the proton and B.sub.0 designates the static magnetic field strength. After termination of the RF pulse, the protons precess in the plane perpendicular to B.sub.0. A sequence of refocusing RF pulses generates a sequence of spin-echoes which produce a detectable NMR signal in the antenna.
U.S. Pat. No. 5,280,243 issued to Melvin Miller describes a nuclear magnetic resonance tool for formation evaluation while drilling. The tool includes a probe section consisting of a permanent magnet disposed in a longitudinally extending annular recess outside the drill collar and an antenna disposed on a non-conductive magnetic sleeve outside the drill collar. The gradient of the static magnetic field magnitude is in the radial direction. The antenna produces an RF magnetic field substantially perpendicular to both the longitudinal axis of the tool and the static field direction. With the '243 apparatus, the magnet must be long in axial extent compared to its diameter for the magnetic fields to approximate its intended 2-D dipole behavior.
U.S. Pat. No. 5,757,186 issued to Taicher et al. describes a measurement-while-drilling tool which includes a sensing apparatus for making nuclear magnetic resonance measurements of the earth formation. The NMR sensing apparatus is mounted in an annular recess formed into the exterior surface of the drill collar. In one embodiment, a flux closure is inserted into the recess. A magnet is disposed on the outer radial surface of the flux closure. The magnet is constructed from a plurality of radial segments which are magnetized radially outward from the longitudinal axis of the tool. The flux closure is required to provide suitable directional orientation of the magnetic field.
The tools disclosed in the '243 and '186 patents suffer from common problems: both tools require using a nonconductive magnet and placing the magnet outside the drill collar. For the '243 tool, the outside surface of the drill collar must contain a recessed area to accommodate the nonconductive magnet. For the '186 tool, the outside surface of the drill collar must contain a recessed area to accommodate the flux closure, nonconductive magnet, and antenna. Because the strength of the drill collar is a function of its radii, reducing the external diameter to accommodate the magnet only or the flux closure, magnet, and antenna results in an unacceptably weak section of the drill collar which may bend or break during the drilling operation.
U.S. Pat. No. 5,557,201 issued to Kleinberg et al. describes a pulsed nuclear magnetism tool for formation evaluation while drilling. The tool includes a drill bit, drill string, and a pulsed nuclear magnetic resonance device housed within a drill collar made of nonmagnetic alloy. The tool includes a channel, within the drill string and pulsed NMR device, through which drilling mud is pumped into the borehole. The pulsed NMR device comprises two tubular magnets, which are mounted with like poles facing each other, surrounding the channel, and an antenna coil mounted in an exterior surface of the drill string between the magnets. This tool is designed to resonate nuclei at a measurement region known to those skilled in the art as the saddle point.
U.S. Pat. No. 5,705,927 issued to Sezginer et al. also describes a pulsed nuclear magnetism tool for formation evaluation while drilling. The tool includes shimming magnets, located either inside or outside the tool, which suppress the magnetic resonance signal of the borehole fluids by raising the magnitude of the static magnetic field in the borehole so that the Larmor frequency in the borehole is above the frequency of the oscillating field produced by an RF antenna located in a recessed area of the tool. The shimming magnets also reduce the gradient of the static magnetic field in the region of investigation.